While waking cognition serves to facilitate an organism's ongoing interactions with the external (physical and social) environment, sleep serves two potentially critical support functions: (1) the restoration and normalization of neurocognitive functions that were engaged and altered during prior waking, and (2) the consolidation, modification, and integration of information encoded during prior waking experiences. There is now considerable evidence that sleep plays important roles in both memory consolidation and coping with emotional stress. Neurocognitive function during waking normally occurs within conscious awareness, and conscious experience can provide evidence of underlying brain processes. Carried over into sleep, we hypothesize that what reaches conscious awareness during sleep (e.g., in dreams) is a reflection of underlying functional brain processes. The study and manipulation of sleep mentation can thus contribute to our understanding of these sleep-dependent neurobiological processes and help us further define the role of sleep in neurocognition and organismic adaptation. Recent advances make it possible to experimentally manipulate and objectively quantify the content of sleep mentation. Using these newly developed techniques, we have shown that images of the video games Tetris and Alpine Racer are incorporated into up to 42% of subsequent sleep onset mentation reports, with as many as 89% of subjects reporting such images. Both visual and kinesthetic sensations are reproduced at sleep onset, but details characteristic of episodic memories, such as the computer, keyboard, desk or room in which Tetris was played, are absent. Tetris imagery was also reported by dense amnesiacs with bilateral medial temporal lobe and hippocampal damage, demonstrating declarative, episodic memories are not required for formation of these sleep onset images. In addition, pilot studies with Alpine Racer have shown that when subjects are awakened at sleep onset later in the night, game imagery disappears and is replaced by more weakly related imagery, such as of falling down a grass covered hill. The studies proposed here use reports of sleep mentation to provide support for the hypothesis that, during the night and across multiple sleep stages, waking memories are reprocessed through a series of distinct steps that aid in their consolidation and integration.